The present invention concerns, generally, tractors for feeding or advancing web material such as edge perforated webs of paper. As such, the devices of the present invention are especially suited to drive webs of paper such as those employed in computer printout equipment, teletypewriters, optical character readers and the like. However, the tractors are capable of broader application, in general for feeding a web of paper-like material in either continuous web or individual sheet form.
Devices adapted to this purpose are, of course, old and well-known in the art. Generally, such tractors include an endless belt trained in a loop over sprocket wheels which are driven by a drive shaft of the equipment (printer, etc.) on which the tractor is mounted. The belt typically has pins projecting from it to engage perforations provided in the paper or other material to drive the material along a linear path. The belt is trained over supports to curve away from the linear path and then return along the loop back to the linear path. Such tractors are normally employed in pairs at opposite edges of the web of material to drive it from both its opposite edges.
One problem associated with such prior art devices is that of maintaining proper tension across the web transversely of its drive direction. This requires precise positioning of the paired tractors laterally of the web. Generally, the prior art has attempted to deal with this problem by shifting the tractors along a support rod on which they are mounted. U.S. Pat. No. 2,140,028 discloses a device in which rotation of a crank turns a threaded shaft to shift paired tractors to adjust the spacing between them. Both tractors are shifted simultaneously by moving the entire tractor along the shaft. U.S. Pat. 3,006,520 shows a somewhat similar device in which the tractors may be shifted individually or simultaneously. U.S Pat. No. 1,938,531 shows a similar concept employed on a typewriter and U.S. Pats. 2,278,565; 3,115,491; 3,152,742; 3,154,235' and 3,688,959 show various means to facilitate such lateral shifting of tractors.
All of these devices involve shifting the entire tractor, are difficult and in some cases impossible of providing such shifting during operation, and do not provide for very small increments of shifting.
Another difficulty associated with prior art devices is that if the drive tractor is to be shifted the operator must do so by either unlocking the tractor mount and attempting to reposition the tractor by hand in the case of slide-mounted tractors; or by advancing the tractor along a threaded shaft as by turning a crank, in the case of threaded-shaft mounted tractors. In the latter case, the threads must be coarse enough to move the tractors at a reasonable rate of speed for initial positioning and therefore fine adjustments of position which require a fine threaded shaft, are not readily attainable. In the former case, accurate hand positioning is difficult particularly if the web material is engaged with the drive tractors while the adjustment is being attempted to adjust web tension.
It is accordingly an object of the present invention to overcome the foregoing and other shortcomings of the prior art by providing a novel drive tractor construction and a novel assembly for driving web material.
It is another object of the present invention to provide a novel and efficient drive tractor construction which permits lateral shifting of individual tractors in exceedingly small increments without necessity of shifting the entire tractor on its support rod.
It is another object of the present invention to provide a novel drive tractor as described above and a novel assembly for driving web material including at least a pair of such drive tractors having a base member lockable upon an elongated support bar and a chassis movable on the base member.
Other objects and advantages of the present invention will become apparent from the following description thereof.